I. Field of the Invention
The present invention relates generally to the field of orthopedic surgery and has particular application to a total artificial joint and the implantation thereof.
II. Description of the Prior Art
This invention relates broadly to means for transferring forces imposed on load bearing portions of artificial joints to bone in humans and animals. While the present invention is applicable for use with implants of various types and in numerous applications in human and animal joints, it will be described herein, for purposes of example only, as being specifically adapted for use in transferring the load on the femoral head replacement of a total hip joint prosthesis, such as the Charnley type, to the femur. Although hip prostheses will be used for illustrative purposes only, features of the invention will be stated in a generic form so that they are applicable to all joint prostheses and to the geometric and biomedical properties of all animal and human joints.
The artificial hip joint is intended as a replacement for both parts of the human hip joint. The natural hip joint consists of a ball-like member at the head of the femur or thighbone, this member being rotatable in a socket, termed the acetabulum, in the pelvis. When this joint becomes damaged or diseased, it is the practice to replace the femoral head with a prosthesis including a ball member attached to the femur by a neck and stem which fits into the medullary canal, and to fit a corresponding artificial socket member into the acetabulum, which may be suitably enlarged for the purpose.
Most of the currently available total joint replacements are comprised of a metal component articulating against a plastic component. For example, the spherical articulating, or bearing, surfaces of the typical prosthetic hip joint include an implanted femoral component having a spherical surface of cobalt/chromium alloy articulating against an acetabular component, or cup, made of high-density polyethylene.
In the late 1960's, Sir John Charnley experimented with the use of tetrafluoroethylene, commonly known by the trademark TEFLON, for the surface of the acetabular component, working against the usual metal component, but the results were less than satisfactory. While TEFLON material in its bulk form is inert in the body, the TEFLON wear debris generated by the articulating surfaces rubbing against one another caused severe histiocytic reactions in the surrounding tissue with resulting trauma and pain experienced by the patient. The reaction was such that the TEFLON components eventually required removal.
More recent hip prostheses have utilized acetabular sockets of high density polyethylene, and it has been the practice to utilize a bone cement, polymethyl methacrylate, for example, to retain the implanted component. There is some uncertainty as to which of the two polymeric materials has been the major source of wear particles that have caused observed adverse tissue reactions, although it has generally been assumed to be the bone cement.
As the technology has progressed, porous coatings have begun to replace bone cement in a high proportion of implantations. This permits natural bone ingrowth, eliminating the need for bone cement and avoiding problems caused by the cementitious wear particles. This new technology has not been in use long enough to determine the extent of problems resulting from wear particles resulting from the interaction between the metal component and the plastic component. However, as prostheses remain in use for longer periods and are used for younger and more active patients, it is inevitable that wear particles from the polyethylene articulating surfaces will become a problem. Hence some means of isolating these particles from the surrounding cellular tissue would be highly beneficial.
For example, there is a substantial and growing body of evidence to the effect that wear particles lead to prosthetic loosening and eventually to catastrophic failure of the prosthesis. In a paper published in the American Volume of The Journal of Bone and Joint Surgery, Vol. 65-A, No. 5 (June 1983) on page 575, Goldring et al state:
"CLINICAL RELEVANCE: This transformation of tissue at the bone-cement interface in patients with a non-septic, loose total hip component to a synovial-like tissue with the capacity to generate 10 prostaglandin E.sub.2 and collagenase may explain the progressive lysis of bone that is seen in some patients with loose cemented total joint implants. Loosening of the component may be a stimulus to the synthetic activity of this tissue, which leads 15 to further resorption of bone. Understanding and the possibility of pharmacological control of this membrane may contribute to improved duration of total joint implants."
It was with knowledge of the current state of the art and in view of the problems previously encountered as noted above, that the present invention was conceived and has now been reduced to practice.